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60GHzCapacitivelyProbe-FedPatchArrayswithSuspendedElementsKavehKeshtkaranandNimaGhalichechianElectroScienceLaboratory,Dept.
ofElectricalandComputerEngineeringTheOhioStateUniversity,Columbus,Ohio,USAEmail:keshtkaran.
2@osu.
edu,ghalichechian.
1@osu.
eduAbstract—Amajordrawbackofcurrentmillimeter-wavetechnologiesusedforintegrationofphasedarraysonachipislowefficiency(5-10%)andconsequentlylowrealizedgain.
Inthiswork,wepresentintegratedantennaarraysonsiliconthatexhibitradiationefficiencyof>80%at60GHz.
Thisisachievedbysuspendingtheradiatingelementsofaphasedarrayinairusingmicro-electro-mechanicalsystems(MEMS)processes,effectivelyreplacingalossysiliconsubstrate(undereachelement)withair.
Inthelatestdesignweusedcapacitivefeedingwithpinandpatchheightof40and60m,respectively.
Finiteelementsimulationresultsverifytheperformanceofthearray.
Afinitearraywith5*5elementsachieved-10-dBbandwidthof1.
7GHz.
Arrayiswellmatchedat60GHzwithS11Suspended,MEMS,PhasedArray,HighEfficiency.
I.
INTRODUCTIONConservativeestimatespredictthatcellulardatatrafficwillgrow40-70%annuallyintheforeseeablefuture,implyingtheneedfornetworkstosupportgreaterthan1000timesthecurrentdatatraffic[1-3].
Inrecentyearstherehasbeengreatinterestin60GHzantennasduetolargeunlicensedbandsavailableat57-64GHz[4].
Thisbandisagreatcandidatefornext-generationshort-rangecommunicationlinks.
However,thereareseveralchallengesforsuccessfulrealizationofmillimeter-wavecommunicationsystems.
Onesuchchallengeisthatthesignalpropagationatmillimeter-wavefrequenciesisimpairedbyseverepath-lossandshadowingeffects[5].
Transmitandreceivebeamformingnetworkswithmany(e.
g.
,≥100)antennasperterminalarenaturalapproachtocounteringtheincreasedpathlossat60GHzband.
Asaresult,nextgenerationantennasoperatingatthisbandneedtobecapableofelectronicscanningwhileexhibitingahighgain.
Currentlytherearetwoapproachesforon-chipantennas.
Inthefirstapproach,theantennaispositionedonthesubstrateresultinginmassiveradiationlosses.
Thisisduetolowresistivityofsiliconcausingmostofthefieldcoupletosiliconsubstrate(withdielectricconstantof11.
7)insteadofradiatinginfreespace.
Improvementstoefficiencyarepossiblebythinningdownthesubstrateorusinghigh-resistivitysubstratewhicharebothundesirableoptionssincetheyarelimitedandcostly.
Despitetheseimprovements,theantennaradiationefficiencyisintheorderof5-10%orless[6-8].
Thestateoftheartapproachutilizesagroundplaneonthesubstratewithathinlayerofsilicondioxide(SiO2)(e.
g.
5minthickness)separatingaradiatingelementsfromthegroundlayer[9].
Duetocloseproximityofthetransmissionlineandradiatingelementstothegroundplane,theconductivelossesdominateresultinginantennaradiationefficiencyintheorderof45%orless.
ThekeylimitationhereisfinitethicknessofSiO2layerinastandardBi-CMOSprocessesusedforfabricatingactivecomponentssuchasT/Rmodules.
Toavoidcrackinginthethickdielectriclayer,metalfences(vias)aredesignedandfabricatedwithinthedielectriclayerthatcontributetoadditionallosses.
Furthermore,highersilicondioxidethickness(betweenthegroundplaneandtheantennaelements)increasesthefabricationcostoftheantennaarray.
Incontrasttotheaforementionedapproachesinrealizingintegratedphasedarrays,thispaperpresentsanovelarchitecturethatusesMEMSsuspendedradiatingelementstogetherwithcapacitively-fedpatchtoachieve>80%efficiency.
Thisapproachhasafewuniquefeatures.
Forinstance,bysuspendingthepatch,theeffectivedielectricconstantofthesubstrateisreducedto1.
Asaresultbyreducingconductive,dielectric,andsurfacewavelosses,theefficiencyoftheantennaisincreased.
Moreimportant,byreducingeffectivedielectricconstant,thearrayisabletoscanmuchlargervolumecomparedtoconventionalpatcharrayantennas.
Wehavealsoimprovedonourpreviousworkthatusedaperturecoupledmicrostripfeednetwork[10].
Unlikeourpreviousdesign,thepin/capacitorfeedingschemeprovidesbettercompatibilityandeasiermonolithicintegrationwithaCMOST/Rsubstrate.
Thispaperisstructuredasthefollowing.
InSectionII,basicdesignandarchitectureofthephasedarrayisdiscussed.
FabricationprocessispresentedinSectionIII.
Simulationresults–includingimpedancematching,efficiency,andscanning–arereportedinSectionIV.
II.
PHASEDARRAYARCHITECTUREA.
UnitCellDesignSuccessfulimplementationofthenext-generationantennaarrayat60GHzwilldependonasimple–yet201711thEuropeanConferenceonAntennasandPropagation(EUCAP)978-88-907018-7-0/17/$31.
002017IEEE#15703175292511important–factor:EaseofintegrationoftheantennaandthesubstratethatholdstheRFfront-endcircuits.
Asmentionedearlier,inatraditionalapproach,theproximityoftheradiatingelement(patch,dipole,etc.
)toalossyhighdielectricconstant(silicon)substrate(orthegroundplane)isamajorsourceofradiationloss.
Incaseswherethesubstrateisshieldedbyagroundplane,alayerofsilicondioxideisusedforseparationbetweentheradiatingelementandthegroundplane[11].
Giventhesizeofthewavelength(λ=5mmat60GHz)andcurrenttechnologylimitationstofabricatethickSiO2layer,themaximumpossibleoxidethickness(5-15m)isstillwellbelowtherequiredthicknesstoavoidohmiclossesandachievehighradiationresistance(e.
g.
λ/10≈500mforapatcharrayat60GHz).
Toaddresstheaforementionedshortcoming,weproposeanovelsuspendedphasedarraystructurethatimprovesefficiencyandscanningperformanceofthearraywhilemaintainingtherequiredbandwidth.
TheunitcellschematicofthesuspendedpatcharrayisshowninFig.
1.
Asillustrated,thepatchissuspended60mabovethegroundplanewithathickSU-8postsdefinedbyaphotolithographyprocess.
Thesepostsoccupyasmallarea,thus,haveaminorimpactontheradiationpatternofthepatch.
WehaverecentlycharacterizedtheelectricalpropertiesoftheSU-8atmillimeterwaveandterahertzbands[12].
Theradiatingelementcanbefabricatedonathinmembraneor–asshowninFig.
1–onathickdielectricsuperstrate.
Unitcellsizeis3mm*3mm.
Thepatchisfedwitha40-m-heightpinformingacapacitivescheme.
EachpinisfeddirectlybyaT/Rmodulelocatedunderneathelements.
ThepinsarefabricatedbymetallizationofthesecondsetofSU-8posts.
B.
FiniteArrayDesignSchematic3DviewofthesuspendedphasedarrayisshowninFig2.
Thearraysizeischosentobe5*5fortheeaseofsimulation,fabrication,andtesting.
Thearraysizeis15mm*15mm.
Asmentionedearlier,thisarchitectureissuitableforactiveelectronicallyscannedarrays.
Largerarraysizescanalsobeconsideredinfuturetoachieveahighergain.
Fabricationandsimulationresultsarereportedinthenextsections.
III.
FABRICATIONThefabricationprocessofthearrayisasfollowing.
First,thefeedlineswerefabricatedbypatteringa1-m-thickgoldlayeronasiliconsubstrate.
A3-m-thickSiO2layerwasthendeposited,patterned,andetchedtoformapinslot.
Next,thegroundplanewaspatternedusingagoldlayer.
Furthermore,40-m-thickSU-8photoresistwasspincotedandpatternedtoformthepostsforpins.
Then,1mconformalgoldlayerwasdepositedandetchedtoformcapacitivecaps.
Onaseparate100-m-thickquartzsubstrate,a1mgoldlayerwasdepositedandpatternedfollowedbyspincoatingandpatterninga60mthickSU-8layertoformthepostsforsuspendedpatch.
Lastly,thetwowaferswerealignedandbondedtogethertoformthefinalarraystructure.
IV.
SIMULATIONRESULTSANSYSHFSSwasusedforthesimulationoftheunitcellofaninfiniteanarray.
Wealsousedthesametoolforthesimulationofthefinite5*5elementarray.
Theimpedancematching(atbroadside)isshowninFig.
3.
Theantennaiswellmatchedat60GHzwithS11suspendedpatcharray.
Fig.
2:3Dschematicofhigh-efficiencyphasedarraywith25elements.
Eachpatchissuspendedon5postsandfedbycapacitivepin.
201711thEuropeanConferenceonAntennasandPropagation(EUCAP)#15703175292512maximumrealizedgainof20dBiatbroadside.
Dependingontheapplication,thegaincaneasilybeincreasedbydesigningalargerarray.
Thearrayiscapableofscanningdownto45°inbothEandHplanes.
Comparedtothebroadside,thegainisreducedby4dBat45°.
Thesidelobesareacceptableandareabout13.
3dBlevel.
Totalradiationefficiencyofthearrayiscalculatedtobe89%.
Fig.
3:Simulationresultsshowingreflectioncoefficient(S11)asafunctionoffrequency.
MinimumS11is19dBandbandwidthisapprox.
1.
7GHz.
Fig.
4:SimulationresultsfortheantennapatternshowingrealizedgainasafunctionofscanningangleforE-plane(top)andH-plane(bottom).
V.
CONCLUSIONInthispaperwepresentedanewdesigntoimproveon-chipphasedarrayantennaefficiencyandrealizedgainat60GHz.
Thisisachievedbypolymer-corecapacitively-fedandsuspendedradiatingelements.
Achievedgainis20dBiwith13.
3dBsidelobeslevel.
The5*5(25elements)arrayiscapableofscanning±45°inEandHplanes.
Inthisdesign,antennaisdirectlyfedfrombelow.
Theareaunderthegroundplanecanbeusedforfront-endelectronics.
Thissavesvaluablesemiconductorspace.
Thisantennawillbetestedbyterminatingallbutthecenterelement.
ThelatterwillbeexcitedbymicrostriplineandRFprobes.
Furtherenhancementstothebandwidth,efficiency,andscanningperformanceisalsopossiblebyreducinggratinglobesandterminatingthefieldsattheedgeofthearray.
Design,simulation,fabrication,andmeasurementresultswillbepresentedattheconference.
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